The present invention relates to imaging systems such as x-ray equipment, computed tomography imaging apparatus and magnetic resonance imagers, and more particularly, to techniques for processing image data to eliminate the effects of movement of the patient between multiple images.
In some computed tomography (CT) systems an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a patient undergoing diagnostic evaluation, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the patient, and each detector produces a separate electrical signal that is a measurement of beam attenuation along a specific ray path. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array mounted on a gantry in a common type of CT system are rotated around the patient so that the angle at which the x-ray beam intersects the patient constantly changes. The gantry may stop or continue to move as the measurements are being made. The images produced from the scan data correspond to a stack of two-dimensional slices taken through the patient.
Typical CT systems may be operated in either the axial mode or the helical scan mode. In the typical axial mode, the patient being imaged remains stationary during each scan. The patient may be moved between rotations in order to obtain different slices through the patient. In the conventional helical scan mode, the gantry with the x-ray source and detector array revolves continuously while the patient is translated through the imaging plane. The data are processed subsequently to form the desired image planes through the patient.
The resultant set of projections from a scan are used to reconstruct images which reveal the anatomical structures at the position of each slice taken through the patient. The prevailing method for image reconstruction is referred to in the art as the filtered back-projection technique. This process converts the attenuation measurements from a scan into an array of integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In many imaging applications, it is desirable or necessary to collect scan data sets for two distinct images of the same subject. For example, in medical applications such as angiography, at least two x-ray image data sets are commonly obtained for a region of tissue being analyzed such as the patient's head. Before the second data set is obtained, a vascular contrast-enhancing agent is injected into the patient. In principal, a process known as image subtraction, or specifically digital image subtraction, can then be applied to the two data sets to "subtract" one image data set from the other to remove background features common to both data sets such as bone and soft tissue, leaving an image data set for only the target tissue of interest, namely, the blood vessels in the region of tissue.
In reality however, the two image data sets generally differ by more than simply the enhanced contrast of the blood vessels. Movements in the region of tissue between images caused by pulsation of vessels, muscle contractions, etc., cause shifts in position of both background tissue and the target tissue of interest. Because of the shifts in position, a simple straightforward subtraction of the image data sets does not produce an accurate image of the target tissue.
Some prior imaging systems and processes have made attempts at reducing the effects of patient movement between images. One of these involves identifying certain "fiduciary points" and tracking their movement between images. For example, when imaging the head of a patient, fiduciary points may include data points defining the tip of the noise, earlobes, teeth, etc. The movement of these fiduciary points between images is identified, and an estimate of the movement of other points between the fiduciary points is generated using interpolation.
This method can produce inaccurate results. The fiduciary points are selected because they are easy to identify in the image data sets, not necessarily because their movement is representative of the movement of all points in the tissue. The points in the tissue between the fiduciary points can move very differently than the fiduciary points themselves. Thus, interpolating between fiduciary points will not produce an accurate measure of the movement of other points in the tissue.
Thus, there is a need for an object or tissue imaging system and method which can accurately compensate for movement of the object or tissue between image data set acquisition.